1. Field of the Invention
The present invention relates to a paper discharge tray that is mounted in an image forming apparatus, such as a copier; printer, or facsimile, and holds paper discharged.
2. Description of Related Art
An image forming apparatus, such as a copier or laser printer, is provided with a paper discharge tray for discharging: already processed paper having a desired image formed thereon by an electrophotographic process; or a document whose image has been read after conveyed to a document reading portion by a document conveying device. FIG. 6 is a schematic cross section showing the structure of a conventional paper discharge tray to be used as a discharge destination of already processed paper having an image formed thereon. In FIG. 6, a paper discharge roller 1 is rotated by a motor, not shown, in a direction indicated by an arrow A. Then through a discharge outlet 5, the paper discharge roller 1 discharges, in a direction indicated by an arrow B, paper 4 conveyed through a paper conveyance path 3 together with a paper discharge roller 2 which is driven into rotation by being brought in contact with the paper discharge roller 1 with a predetermined pressure.
The paper 4 discharged is sequentially loaded onto the paper discharge tray 6. The paper discharge tray 6 includes: a paper loading portion 7 having a first tray surface 7a for receiving the substantially front portion of paper discharged and a second tray surface 7b for receiving the substantially rear portion of paper discharged arranged in a bending manner; a rear wall 8 for holding and aligning the trailing end of paper; and a front wall 9 for holding the leading end of paper to prevent the paper from dropping off from the paper loading portion 7. To ensure a certain paper stack amount, there is provided a height difference h between the discharge outlet 5 and the paper loading portion 7 of the paper discharge tray 6.
The paper loading portion 7 is generally so arranged as to incline upward in the paper discharge direction (direction indicated by the arrow B in the figure) so that the paper 4 discharged through the discharge outlet 5 will not drop off from the paper loading portion 7. In this embodiment, the second tray surface 7b is so arranged as to incline upward more steeply than the first tray surface 7a so that the paper loading portion 7 is bent into a substantially inverted-V shape. Thus, paper discharged is loaded along the bending direction, thereby absorbing the curling tendency (upward curling) of the discharged paper. The inverted-V-shaped bending form controls the bulk height of the paper discharge tray 6 and also contributes to space saving at a time of loading the paper discharge tray 6 in an image forming apparatus.
In such a paper discharge tray, in accordance with paper size within a predetermined size, the height difference h provided for ensuring the paper stack amount and also the slope of the paper loading portion 7 are both fixed at predetermined dimension and predetermined orientation. This permits loading and then stocking paper of a predetermined size or a size nearly equal to the predetermined size without any problems. Therefore, when the height difference h is larger than the paper size, for example, the leading end portion of the paper 4 discharged through the discharge outlet 5 droops so largely as to come into substantially vertical contact with the paper loading portion 7, as shown in FIG. 7A. Further discharge of the trailing end portion of the paper 4 in this state results in a risk that the paper 4 is inverted, as shown in FIG. 7B. On the other hand, a smaller height difference h results in a failure to ensure a sufficient paper stack amount.
A smaller height difference h results in smaller upward inclination of the second tray surface 7b in the paper discharge direction. Therefore, depending on the paper discharge speed, the leading end of the paper 4 protrudes toward the first tray surface 7a side beyond the bending portion. Since the inclination of the first tray surface 7a is more moderate than that of the second tray surface 7b, the friction between loaded paper sheets prevents the paper 4 from slipping down to the rear wall 8, thereby leading to a disturbance in the loading condition, particularly when the paper 4 is discharged continuously. This phenomenon becomes more outstanding with an increase in the number of paper discharged. The description has been given here, referring to a paper discharge tray to be used as a discharge destination of paper having an image already formed thereon. However, the description is also similarly applicable to a paper discharge tray to be used as a discharge destination of a document automatically conveyed by the document conveying device.
Various methods have been proposed which properly load paper of different sizes on a discharge tray while ensuring a sufficient paper stack amount. For example, Japanese Patent Application Laid-open NO. H5-97313 (hereinafter referred to as patent document 1) discloses a paper discharge tray capable of loading paper on appropriate position regardless of the size and stiffness of the paper by providing a mountain-like guide that is movable in the paper discharge direction. Moreover, Japanese Patent Application Laid-open No. 2004-35214 (hereinafter referred to as patent document 2) discloses a paper discharge tray including a paper discharge board and a movable loading board. The paper discharge tray disclosed in the patent document 2 adjusts the height difference from the paper discharge outlet to the paper discharge board by transferring the movable loading board in the paper discharge direction in accordance with the paper size, thereby ensuring the paper stack amount in accordance with the paper size.
However, the method disclosed in the patent document 1 is not intended to ensure the paper stack amount. Thus, this method can load paper of various types properly on their respective predetermined positions regardless of the sizes and hardness of the paper, but fails to provide a sufficient paper stack amount. The method disclosed in the patent document 2 requires the entire paper discharge tray to be moved in accordance with the paper size, thus resulting in complicated structure of the paper discharge tray and cumbersome operation to be performed by the user.